liminfoAspen Plus Reference
Free reference guide: Aspen Plus Reference
About Aspen Plus Reference
The Aspen Plus Reference is a practical quick-lookup guide for AspenTech's Aspen Plus process simulation software. It covers the complete simulation workflow from flowsheet setup (component input, unit settings, stream specification) through property method selection to equipment modeling and analysis tools. Property method entries provide selection criteria: Peng-Robinson and SRK for non-polar hydrocarbons, NRTL/UNIQUAC/WILSON for polar non-electrolyte systems, ELECNRTL for electrolytes, and GRAYSON/BK10 for petroleum refining applications.
Equipment model entries include mixers/splitters (FSplit, SSplit), heat exchangers (HeatX with shortcut and detailed shell-and-tube modes, LMTD calculation), simple heaters/coolers, flash drums (Flash2 for VLE, Flash3 for VLLE, Decanter for LLE), pumps, and compressors (isentropic/polytropic with multi-stage intercooling). The distillation section covers RadFrac (rigorous MESH equations with design specs for purity/recovery), DSTWU (Winn-Underwood-Gilliland shortcut), and absorber/stripper configuration using RadFrac without condenser/reboiler, including RateSep for rate-based non-equilibrium modeling.
Reaction and analysis tool entries cover RGibbs (Gibbs energy minimization for combustion/reforming/gasification), RStoic (stoichiometric with conversion), RYield (mass yield for complex reactions), RCSTR (perfectly mixed), and RPlug (plug flow with Power Law and Langmuir-Hinshelwood kinetics). Analysis tools include Sensitivity analysis for parameter sweeps, Design Spec for back-calculation (e.g., adjusting reflux ratio to hit 99% purity), Calculator blocks for custom expressions, convergence settings (Wegstein, tear streams), energy/pinch analysis, property analysis (Txy, Pxy, residue curve maps), optimization (SQP), and economic evaluation (CAPEX/OPEX/NPV/IRR).
Key Features
- Property method selection guide: Peng-Robinson, SRK, NRTL, UNIQUAC, WILSON, ELECNRTL, GRAYSON, BK10 with application criteria
- NRTL/UNIQUAC activity coefficient models with binary interaction parameters and UNIFAC group contribution estimation
- Equipment models: Mixer/Splitter, HeatX (shortcut/detailed), Heater/Cooler, Flash2/Flash3/Decanter, Pump/Compressor
- RadFrac rigorous distillation: stage count, feed stage, condenser/reboiler type, reflux ratio, design specs for purity and recovery
- DSTWU shortcut distillation: Winn-Underwood-Gilliland method for minimum stages, minimum reflux, and actual stage estimation
- Reactor models: RGibbs (equilibrium), RStoic (stoichiometric), RYield (yield-based), RCSTR (CSTR), RPlug (PFR with kinetics)
- Analysis tools: Sensitivity analysis, Design Spec back-calculation, Calculator blocks, Optimization (SQP), and Energy/Pinch analysis
- Convergence settings (Wegstein/Broyden/Newton), tear stream management, petroleum assay input with pseudo-component generation
Frequently Asked Questions
How do I choose the right property method in Aspen Plus?
For non-polar hydrocarbons and gases, use Peng-Robinson or SRK equations of state. For polar liquid mixtures (alcohols, ketones, amines), use activity coefficient models like NRTL (best for LLE), UNIQUAC, or WILSON. For electrolyte systems, use ELECNRTL. For petroleum refining, use GRAYSON (with hydrogen) or BK10. High-pressure systems work best with cubic equations of state (PENG-ROB, SRK, BWR-LS).
What is the difference between RadFrac and DSTWU?
RadFrac is a rigorous model solving the full MESH (Material balance, Equilibrium, Summation, Heat balance) equations stage-by-stage, suitable for detailed design and optimization with design specs. DSTWU uses the Winn-Underwood-Gilliland shortcut to quickly estimate minimum stages (Nmin), minimum reflux ratio (Rmin), actual stages, and feed stage location. Use DSTWU for preliminary design and RadFrac for final detailed simulation.
When should I use RGibbs vs RStoic vs RPlug?
RGibbs minimizes Gibbs free energy to find equilibrium composition without needing reaction equations, ideal for combustion, reforming, and gasification. RStoic requires explicit reaction stoichiometry and conversion, suitable for well-defined reactions. RPlug models plug flow reactors with kinetic rate expressions (Power Law, Langmuir-Hinshelwood) and temperature profiles. Use RCSTR for perfectly mixed reactors with kinetics.
How do I use Design Spec in Aspen Plus?
Design Spec performs back-calculation by adjusting a manipulated variable to meet a target specification. Define the target variable (e.g., distillate mole fraction), set the specification value (e.g., 0.99), choose the variable to manipulate (e.g., reflux ratio), and set the search range (e.g., 1.0 to 10.0). Aspen uses convergence methods (Secant, Wegstein, Newton) to find the solution iteratively.
What is a tear stream and why does convergence matter?
A tear stream is where Aspen "cuts" a recycle loop to solve iteratively. The solver guesses initial values for the tear stream, calculates the flowsheet, compares results with the guess, and iterates until convergence (tolerance typically 0.0001 on flow). Wegstein is the default method for fast convergence. If convergence fails, try improving initial estimates, changing the tear stream location, or applying a damping factor.
How do I perform sensitivity analysis?
Go to Model Analysis Tools > Sensitivity. Define the independent variable to vary (e.g., reflux ratio R/D), set the range and step size (e.g., 1.0 to 5.0, step 0.5), and tabulate the dependent variable (e.g., distillate purity). Aspen runs the simulation at each point and generates a results table and plot. This is essential for understanding parameter effects before optimization.
How does HeatX differ from a simple Heater block?
A Heater block is a single-stream utility that specifies outlet temperature and pressure, calculating duty as the result. HeatX models a two-stream heat exchanger with hot and cold sides. In shortcut mode, you specify UA or an outlet temperature. In detailed mode, you input shell-and-tube geometry for rigorous heat transfer coefficient calculation. HeatX computes LMTD, correction factors, and required area.
How do I handle petroleum crude in Aspen Plus?
Use Properties > Petroleum to input assay data: TBP (True Boiling Point) or ASTM D86 distillation curve, API gravity, and sulfur content. Aspen generates pseudo-components at specified cut point intervals (10-50C), each characterized by Tb, MW, and SG. Use BK10 or GRAYSON as the property method. This approach is standard for crude oil refining and vacuum distillation simulations.